A wide variety of roller cone drill bits have previously been used to form wellbores in downhole formations. Such drill bits may also be referred to as “rotary” cone drill bits. Roller cone drill bits frequently include a bit body with three support arms extending therefrom. A respective cone assembly is generally rotatably mounted on each support arm opposite from the bit body. Such drill bits may also be referred to as “rock bits”.
A wide variety of roller cone drill bits have been satisfactorily used to form wellbores. Examples include roller cone drill bits with only one support arm and one cone, two support arms with a respective cone assembly rotatably mounted on each arm and four or more cones rotatably mounted on an associated bit body. Various types of cutting elements and cutting structures such as compacts, inserts, milled teeth and welded compacts have also been used in association with roller cone drill bits.
Cutting elements and cutting structures associated with roller cone drill bits typically form a wellbore in a subterranean formation by a combination of shearing and crushing adjacent portions of the formation. Roller cone drill bits often operate at relatively low speeds with heavy load applied to the bit. This produces very high loads on the associated bearing structures, increasing wear on the bearing structure and directly impacting the life of the bearing. In many cases, bearing life determines bit life. Therefore, design of bearing structure is often a key issue for roller cone bit manufacturers.
Three major types of bearings are frequently used in the roller cone bit industry: journal bearings (also referred to as a friction bearing), roller bearings and solid bearings. The arrangement and configuration of bearings associated with a roller cone drill bit may be referred to as a “bearing system,” “bearing assembly” or “bearing structure.”
A roller bearing system includes one or more rollers. For example, one type of roller bearing system is a roller-ball-roller-roller bearing structure. Other roller bearing systems incorporate various combinations of roller and ball bearing components and may include, for example, a roller-ball-roller structure or a roller-ball-friction structure. With only limited space available in a typical roller cone assembly for a bearing structure, the proper balance between the size of roller and ball bearings must be maintained in order to prevent excessive wear or premature failure of any elements.
A journal bearing, which has been implemented into roller cone bits since approximately 1970, typically includes a journal bushing, a thrust flange and ball bearing. The journal bushing is used to bear some of the forces transmitted between the journal and the cone assembly. The thrust flange typically bears the load parallel to the journal axis (axial load). Efforts have been made to increase the load carrying capability of the bearing including those discussed in U.S. Pat. No. 6,260,635 entitled, Rotary Cone Drill Bit with Enhanced Journal Bushing and U.S. Pat. No. 6,220,374 entitled, Rotary Cone Drill Bit with Enhanced Thrust Bearing Flange.
A solid bearing is similar to journal bearing but does not include the bushings and flange of a typical journal bearing. Instead of using bushing and flange, a wear resistant hard material such as natural and synthetic diamond, polycrystalline diamond (PCD) may be used to increase the wear resistance of associated bearing surfaces.
The design of bearing systems and bearing structures within roller cone drill bits is typically driven by a designer's field observations and years of experience. Load distribution on bearings are usually estimated by assuming the magnitude of the forces acting on associated cutting structures such as rows of teeth and/or inserts. In instances in which the cutting structures of roller cones vary, an assumption is usually made that the design of a bearing structure is suitable for many cutting structures as long as basic characteristics such as bit diameter, bearing angle and offset are the same. Current industry practice is that for a particular of roller cone drill bit, the same size and type of bearing structure may be used for each associated cone assembly.